Gene expression is very accurate because the enzymes involved play an active role in selecting the correct counterpart for template information through a mechanism of proofreading. I propose to isolate and characterize biochemically E. coli mutants altered in accuracy in gene expression. Non-lethal mutants affecting the ribosome or RNA polymerase will be selected directly by their inability to propagate certain bacteriophage. Altered levels of transcription and mistranslation can be monitored on the selection plates by including a sensitive indicator for lac operon expression. In particular, mutants with altered release factors, elongation factors EFTu and EFTs, 30S ribosomal protein S1, RNA polymerase subunits and possibly sigma-factor are expected among the candidates. It is likely that alteration of cell components involved in maintaining accuracy of gene expression will have far reaching consequences for cell physiology. A better understanding of these mechanisms in E. coli will be helpful in evaluating their possible role in eukaryotic cell, particularly with respect to aging and cancer.